1. Field of the Invention
The present invention relates to a traction control system for motor vehicles, and more particularly it pertains to such a system wherein a slip of driving wheels which tends to be caused when the motor vehicle starts or accelerates is detected to control the braking of the driving wheels, thereby restraining the slip of the driving wheels.
2. Description of the Prior Art
Known in the art is a traction control system for motor vehicles, which is arranged, when the motor vehicle runs on a low-friction road surface such as a frozen, snow-covered or muddy one, to restrain a slip of driving wheel which tends to be caused in an attempt to start or accelerate the motor vehicle, thereby enhancing the starting/accelerating performance as well as the running stability of the vehicle. With such a conventional traction control system, it has been the usual practice that a slip of each of the left-hand and right-hand driving wheel is detected; and when the extent of the slip exceeds a predetermined value, a brake hydraulic pressure applied to a brake device for each driving wheel is increased so that the braking of the driving wheels is controlled, while at the same time the engine output is also controlled, as disclosed in Japanese Patent "Kokai" (Laid-opened) Publication No. 61-85248.
To control the brake hydraulic pressure to be applied to the driving wheels, the following procedures are commonly performed: A plurality of threshold levels are set up in relation to the vehicle speed so that a plurality of driving wheel speed ranges are established between adjacent ones of the aforementioned threshold levels; detection is made as to which speed range the current driving wheel speed belongs to, and detection is also made as to whether such driving wheel speed is increasing or decreasing; and buildup, reduction and holding of the brake hydraulic pressure for the driving wheels are controlled on the basis of such detections.
Since the left-hand and right-hand driving wheels are associated with each other through a differential gear, however, with such a conventional control system that the brake hydraulic pressure is increased or reduced in dependence on the aforementioned behavior of the driving wheels, there is a tendency that there occurs such a hunting phenomenon that if one of the driving wheels is rapidly decelerated by being braked, then the other driving wheel is rapidly accelerated so that a great speed difference is repeatedly caused to occur between the left-hand and right-hand driving wheels during a short period of time, thus causing the vehicle body to be vibrated.